[clang] [emacs][clang-format] Add elisp API for clang-format on git diffs (PR #112792)

[Luke Lau via cfe-commits]

https://github.com/lukel97 approved this pull request.

Apologies for the delay on this again. However I tried it out locally and it now seems to work on macOS, thanks for fixing that! 

I really can't speak much for the elisp, but we don't have many reviewers for the emacs stuff and this feature would be really handy to have.

So LGTM, if it needs more review I think it can be done post-commit :)

https://github.com/llvm/llvm-project/pull/112792